Add save svg-badge directly to gist

This adds the posibility of saving an SVG badge generated by the same
shields.io dirictly to the gist. Instead of prepering a JSON file to
be sent to their service, we use their library directly, which outputs
an SVG file that we can save to the user’s gist.

Filenames ending in `.svg` will use this library automatically.

Additionally there is a major refactoring where the older `node:http`
library has been swapped out for `fetch`.

Also swap from node 16 to node 20

fixes #24

node_modules/css-unit-converter/CHANGELOG.md

@@ -0,0 +1,15 @@
+# 1.1.2 - 2020-05-16
+
+* Add TypeScript definitions (thanks to @seaneking)
+
+# 1.1.1 - 2017-01-25
+
+* Change decimal rounding technique
+
+# 1.1 - 2017-01-24
+
+* Add optional precision argument
+
+# 1.0 - 2015-05-18
+
+* Initial release